1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier and a fixing device provided in the image forming apparatus and, particularly to a fixing device for fixing a toner image to a sheet using a heating roller heated by an induction heating method and an image forming apparatus using such a fixing device.
2. Description of the Related Art
In recent years, the use of power-saving fixing devices of the induction heating type has been widely spread due to a reduction in the heat capacities of heating rollers and an improvement in the heating efficiency of heating rollers. In the conventional fixing devices of the induction heating type, it is designed to thin a heating roller as the heat capacity of the heating roller is reduced. However, such thinning of the heating roller reduces an amount of heat transfer of the heating roller in longitudinal direction and, resulting from the improvement in the heating efficiency, the influence of detection lags (delays) of a temperature detecting thermistor and a thermostat in response to a temperature increasing rate of the heating roller increases. This has caused a problem that the temperature of the heating roller partially excessively increases to cause an error in the image fixing operation such as a hot offset.
In order to solve the above problem, various technologies using heating rollers including a temperature-sensitive magnetic metal layer and a nonmagnetic metal layer have been proposed, for example, in Japanese Unexamined Patent Publication No. 2004-151470 (hereinafter, “document D1”). In an induction heating fixing device disclosed in the document D1, an increase in the temperature of the heating roller above a preset Curie temperature of the temperature-sensitive magnetic metal layer is suppressed by regulating the thickness of the temperature-sensitive magnetic metal layer of the heating roller to a specified range.
FIG. 4 is a section showing the construction of a conventional fixing device. With reference to FIG. 4, a conventional fixing device 247 of the induction heating type as disclosed in the above document D1 is provided with a tubular heating roller 247a that produces heat by an induction heating method, a pressure roller 247b disposed in such a manner that can be pressed into contact with the heating roller 247a, and an induction heating coil 247c for increasing the temperature of the heating roller 247a, wherein a toner image on the front face of a sheet passing a contact portion between the heating roller 247a and the pressure roller 247b is fixed to the sheet by the heat produced by the heating roller 247a. The heating roller 247a has a double-layer structure comprised of a temperature-sensitive magnetic metal layer 247d having a width substantially constant along a direction of an axis of rotation of the heating roller 247a and a nonmagnetic metal layer 247e provided on the outer side of the temperature-sensitive magnetic metal layer 247d. The heating roller 247a has the opposite ends thereof rotatably supported by bearings 247f, and a drive gear 247g is mounted at one end of the heating roller 247a. Thus, width L11 of the heating roller 11 is at least larger than a sheet passing area L12 by as much as a sum of areas where the heating roller 247a are in contact with the bearings 247f and an area where the drive gear 247g is mounted.
The induction heating coil 247c is disposed in the heating roller 247a. Since it is sufficient for this induction heating coil 247c to be capable of at least heating a part of the heating roller 247a corresponding to the sheet passing area L12, width L13 of the induction heating coil 247c is slightly larger than the sheet passing area L12 as shown in FIG. 4. Thus, in the conventional fixing device 247 of the induction heating type as shown in FIG. 4, the width L11 of the heating roller 247a is larger than the sheet passing area L12 and the width L13 of the induction heating coil 247c. 
In the conventional fixing device 247 of the induction heating type as disclosed in the document D1, the width L11 of the heating roller 247a including the temperature-sensitive magnetic metal layer 247d is larger than the width L13 of the induction heating coil 247c. Thus, if the temperature of the heating roller 247a is equal to or below the Curie temperature of the temperature-sensitive magnetic metal layer 247d, magnetic fields produced by the induction heating coil 247c are pulled by the temperature-sensitive magnetic metal layer 247d as shown in FIG. 5, thereby diffusing outward. Accordingly, the magnetic fields supplied to portions of the heating roller 247a corresponding to the vicinities of the opposite ends of the sheet passing area L12 weaken, wherefore there is a problem of being difficult to increase the temperatures of the portions of the heating roller 247a corresponding to the vicinities of the opposite ends of the sheet passing area L12. This results in a problem that the toner image is insufficiently fixed at the opposite ends of the sheet having the width L12 during a warm-up period.
In the conventional fixing device 247 of the induction heating type as disclosed in the above document D1, it can be thought to suppress a temperature decrease at the portions of the heating roller 247a corresponding to the vicinities of the opposite ends of the sheet passing area L12 by increasing an amount of a ferrite core at portions of the heating roller 247a corresponding to the opposite ends of the sheet passing area L12 or increasing the number of turns of the coil at these portions. However, in such a case, a new problem of enlarging the parts due to an increase in the used amount of the ferrite core or the coil is created and, in addition, the warm-up period becomes longer and the toner image is likely to be insufficiently fixed at the central side of the sheet passing area since the magnetic fields weaken at the central side of the sheet passing area as the magnetic fields strengthen at the opposite ends of the sheet passing area.